Throughout the present description and in the subsequent claims, the expression “motion transmission system” is used to indicate the assembly of at least some of the components which act for the transmission of the motion, imparted by the cyclist through pedaling, to the rear wheel of the bicycle.
As known, the motion transmission system of a bicycle comprises a pair of crank arms, on which the cyclist exerts a propulsive thrust by pedaling, one or more driving toothed wheels, set in rotation by direct coupling with the crank arms, and one or more driven toothed wheels (hereinafter “sprockets”) coupled with a rear wheel of the bicycle through a hub and set in rotation by the driving toothed wheels through a chain.
The hub comprises a first body rigidly constrained to the rim of the bicycle rear wheel typically through a plurality of spokes, and a second body rigidly coupled with the sprockets and capable of rotating freely with respect to the first body in a direction of rotation and of dragging it in rotation in the opposite direction, thus applying the forward motion to the rear wheel. In the technical jargon, this second body is called a “freewheel body”.
The assembly of sprockets mounted on the freewheel body is commonly called a “sprocket assembly”.
The chain typically consists of a succession of links, each normally consisting of a pair of mutually facing plates spaced apart to define a space for inserting a tooth of a toothed wheel and/or of a sprocket. The plates of a link are rotatably coupled with the plates of the next link through a rivet. Such a coupling is obtained by arranging the end portions of the plates of a link (hereinafter “outer plates” and “outer link”) over the end portions of the plate of the subsequent link (hereinafter “inner plates” and “inner link”) and by inserting the rivet at holes specifically provided on the outer plates and on the corresponding inner plates. A bush is provided around the rivet, such a bush being capable of rotating freely with respect to the rivet. Flange portions of the inner plates are arranged between the rivet and the bush, such flange portions extending towards the inside of the inner link.
A chain having such a geometry will be indicated hereinafter as “conventionally-shaped chain”.
Throughout the present description and in the subsequent claims, the following definitions will be used.
The terms “outer” and “inner” are used with reference to a longitudinal axis of symmetry of the chain, when the chain is arranged open and straight on a plane. Therefore, “outer link” indicates a link whose (outer) plates are distant from the aforementioned axis of symmetry more than the (inner) plates of the “inner link”.
The expression “end portion” of a link or plate of the chain is used to indicate the end portion of the link or plate along a direction parallel to the aforementioned longitudinal axis of symmetry. Such an “end portion” is therefore the portion of link or plate that, when the chain is mounted, is radially juxtaposed to (in the case of an outer link or plate) or arranged between (in the case of an inner link or plate) a portion of link or plate of the subsequent link.
The expression “central portion” of a link or plate is used to indicate the portion of the link or plate arranged between the two opposite end portions of the same link or plate.
The term “thickness” of a plate is used to indicate the thickness taken on a plane perpendicular to the aforementioned longitudinal axis of symmetry. With particular reference to the inner plates, the thickness as defined here is taken on a portion of plate that does not comprise the aforementioned flange portions.
The expression “resistant transverse section” of the chain or link is used to indicate the resistant section of the outer or inner chain or link, taken at the aforementioned perpendicular plane and defined only by the thickness of the plates of the link(s) at said perpendicular plane. The resistant transverse section thus defined does not therefore take into account the contribution given by further structural elements in addition to the links, like for example the bushes arranged around the rivets that join two adjacent links of the chain.
The expression “maximum thickness of the chain” is used to indicate the distance between the outer surfaces of the outer plates of the chain.
Since the bicycle is a transport device based on the muscle propulsion, it is necessary for the system for the power transmission from the cyclist to the driving wheel to be the least tiring possible.
As known, the combination of a toothed wheel with a small diameter and a sprocket with a large diameter allows difficult climbs to be tackled with agility. However, the same combination, in a flat or downhill route, is disadvantageous since it wastes the energy of the cyclist, who is forced to pedal at a fast rate while the bicycle moves forwards at low speed.
In order to adapt the aforementioned combination to the route to be tackled, it is known to provide the bicycle with a plurality of toothed wheels and with a plurality of sprockets, which can be combined with each other depending on the requirements, through suitable gearshifting devices, thus defining a plurality of gear ratios.
With particular reference to sprockets, they are spaced apart from one another by respective spacers. Such spacers have the task of defining a space between two adjacent sprockets which is suitable for allowing the engagement of the chain on a sprocket and the passage of the chain from one sprocket to another.
Over the years the number of sprockets used has progressively increased.
In order to minimize the weight and be able to mount an increasing number of sprockets on the freewheels already on the market (typically provided to house a smaller number of sprockets), the increased number of sprockets has typically been accompanied by a reduction in thickness of the sprockets, and/or of the spacers and/or of the chain.
The Applicant has designed a sprocket assembly comprising twelve sprockets. In order to be able to continue to use freewheels already used with ten or eleven sprockets, the Applicant has reduced the space between each pair of consecutive sprockets, so as to obtain a sprocket assembly (of twelve sprockets) having a length which is compatible with the current construction standards of frames and bicycle components.
The Applicant has observed that the aforementioned sprocket assembly requires the use of a chain having a reduced maximum thickness.
The Applicant has however observed that a reduction of the maximum thickness of the chain could have led to an undesired reduction of the structural strength thereof.
The Applicant has therefore considered how to be able to reduce the maximum thickness of the chain without compromising the structural strength thereof.
In this respect, the Applicant has verified that the structural strength of the chain is mainly influenced by the thickness of the central portions of the plates of the links of the chain. Indeed, it is at such central portions that the Applicant found yielding of the chain during tensile strength and lateral load resistance tests.
The Applicant has therefore realized that it is possible to obtain the desired reduction in maximum thickness of the chain, without compromising the structural strength thereof, by reducing the thickness of the plates of the inner and/or outer links only at the end portions of such links. In this way, it is possible to assemble a chain wherein, being equal the thickness of the central portions of the links (and therefore being substantially equal the structural strength) the two outer plates are space apart from each other at a shorter distance than in a conventional chain, thus obtaining the desired reduction of the maximum thickness of the chain.
The solution identified by the Applicant goes contrary to the tendency often followed in the past of reinforcing (thus providing an increased thickness) the plates of the inner and/or outer links of the chain at the end portions of such links.
The Applicant has in practice reversed this tendency.